Six Is the Floor, Not the Target
Boeseman's rainbowfish are true schoolers, and their behaviour is calibrated to a crowd. Below about six, the school never quite forms: the fish scatter into the planting, startle as individuals, and show a fraction of their colour. At six a loose group appears; at eight to ten in a tank with real swimming length, the transformation completes — the school cruises together in the open, males flare their blue-grey fronts and orange-yellow tails at each other in harmless display, and the whole group feeds boldly at the surface. If the tank can only hold four comfortably, the honest answer is to choose a smaller rainbowfish species rather than under-stock this one.
Why the Males Need Each Other
Colour in this species is a social signal, and it switches on in company. A lone male, or a pair kept by themselves, commonly stays muted for life — owners often believe they were sold dull fish when they actually bought fish with nobody to impress. Two or three males in a mixed school display constantly and harmlessly, the rivalry expressed as parallel swimming and flaring rather than contact. Females, plainer and more olive, are not filler: their presence is half the reason the males perform, and a school weighted too heavily toward males can leave the females over-attended, so a roughly even mix or slightly more females suits a breeding-free display tank.
Length of Tank Over Everything
Boeseman's are among the more powerful swimmers in the rainbowfish family and they use a tank horizontally. A long footprint lets the school build speed, turn, and cruise as a unit; a tall, narrow tank of the same volume frustrates them into pacing the glass. Adult size matters too — these fish grow substantially larger than the slim juveniles in shop tanks, and a school of eight adults is a significant presence that a small aquarium cannot absorb. Plan the school and the tank together: the group size the question asks about is meaningless until it is matched against the adult fish travelling the full length of their home at speed.
Building the School Sensibly
Buy the group in one go or in two close waves, since rainbows added singly to an established school can be picked on briefly while a pecking order resets. Juveniles are pale and sexing them in the shop is genuinely difficult, so a group bought young is partly a lottery; keepers who care about the male-to-female balance often buy a few extra juveniles and rehome surplus males as they colour up, which the display behaviour makes easy to judge. Feed a varied diet with some colour-supporting foods in it, because the orange half of the fish deepens noticeably on good feeding, and dim, tannin-free bright water shows the contrast best.
A School Is a Commitment to Record
A school this active repays simple record-keeping: when the group went in, the eventual sex ratio once colours declared it, and any fish lost, since a school thinned by losses to three or four quietly reverts to the shy behaviour the group size was chosen to avoid. Topping a school back up is normal maintenance for this species, not an admission of failure. Keeping the school's numbers and history in the household's shared notes through KinsLe Family Sharing means whoever notices the group looking sparse has the full picture of what it should be.